Silicon thin film transistor with an intrinsic silicon active layer formed within the boundary defined by the edges of the gate electrode and the impurity containing silicon layer

ABSTRACT

A silicon thin film transistor array includes a plurality of silicon thin film transistors in an array-like form, each silicon thin film transistor including an insulating substrate, a gate electrode formed on the insulating substrate, a gate insulating layer formed on the insulating substrate containing the gate electrode, a pair of first impurity contained silicon layers formed on the gate insulating layer in such a manner as to transversely cross a terminal part of the gate electrode, an intrinsic silicon layer formed on the pair of first impurity contained silicon layers and on the gate insulating layer between the pair of first impurity contained silicon layers in such a manner as to connect the pair of first impurity contained silicon layers, a protective insulation layer formed on the intrinsic silicon layer, and a source electrode and a drain electrode formed at contact parts of the pair of first impurity contained silicon layers; gate wiring for connecting the gate electrodes of the silicon thin film transistors to each other; and source wiring for connecting the source electrodes of the silicon thin film transistors to each other.

This is a division of application Ser. No. 377,873, filed Jul. 10, 1989,now U.S. Pat. No. 5,021,850.

BACKGROUND OF THE INVENTION:

The present invention relates to a silicon thin film transistor utilizedin an active matrix type liquid crystal display device and a method forproducing the same.

Referring initially to FIG. 26, there is shown an example of aconventional silicon thin film transistor which includes an insulatingsubstrate 21, a gate electrode 22 thereon, a gate insulating layer 23 onsubstrate 21 and electrode 22, an impurity contained silicon layer 24containing a suitable amount of impurities which become a donor or anacceptor, an intrinsic silicon layer 25 disposed between gate insulatinglayer 23 and silicon layer 24 and which becomes an active layer, aprotective insulating layer 26, a source electrode 27 and a drainelectrode 28.

As shown in the same Figure, such a silicon thin film transistor inwhich intrinsic silicon layer 25 and protective insulating layer 26 havebeen continuously formed, has excellent reliability, reproducibility ofcharacteristics, and the like, and research and development thereof iswidely carried out thereon.

In the above-described conventional silicon thin film transistor,protective insulating layer 26 formed by use of silicon nitride, siliconoxide, or the like is removed to expose intrinsic silicon layer 25, andan impurity contained silicon layer 24 is formed on this exposedintrinsic silicon layer 25.

However, at the protective insulating layer side of intrinsic siliconlayer 25, there is a layer in which nitrogen, oxygen, or the likecontained in protective insulating layer 26 has diffused. Whenprotective insulating layer 26 is removed, this layer in which nitrogen,oxygen, or the like has been diffused is not completely removed.Accordingly, the junction of the intrinsic silicon layer and theimpurity contained silicon layer becomes insufficient, resulting indeterioration of the transistor characteristics.

The present invention solves the above-described problems by improvingthe junction state of the intrinsic silicon layer and the impuritycontained silicon layer to provide a silicon thin film transistor havinggood transistor characteristics, and a method for producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan diagram showing a first embodiment of the presentinvention;

FIGS. 2(a)-2(c) are cross-sectional diagrams taken along line II--II ofFIG. 1, showing the production procedure of the first embodiment of FIG.1;

FIG. 3 is a plan diagram showing a second embodiment of the presentinvention;

FIG. 4 is a cross-sectional diagram taken along line IV--IV of FIG. 3;

FIG. 5 is a plan diagram showing a third embodiment of the presentinvention;

FIG. 6 is a cross-sectional diagram taken along line VI--VI of FIG. 5;

FIG. 7 is a plan diagram showing a fourth embodiment of the presentinvention;

FIG. 8 is a cross-sectional diagram taken along line VIII--VIII of FIG.7;

FIGS. 9 and 10 are plan diagrams showing a fifth embodiment of thepresent invention;

FIGS. 11(a)-11(c) are cross-sectional diagrams showing the productionprocedure for forming a sixth embodiment of the present invention;

FIGS. 12(a)-12(c) are cross-sectional diagrams showing the productionprocedure for forming a seventh embodiment of the present invention;

FIG. 13 is a cross-sectional diagram showing an eighth embodiment of thepresent invention;

FIGS. 14(a)-14(d) are cross-sectional diagrams showing the productionprocedure for forming a ninth embodiment of the present invention;

FIG. 15 is a plan diagram showing a tenth embodiment of the presentinvention;

FIG. 16 is a cross-sectional diagram taken along line XVI--XVI of FIG.15;

FIG. 17 is a plan diagram showing an eleventh embodiment of the presentinvention;

FIG. 18 is a cross-sectional diagram taken along line XVIII--XVIII ofFIG. 17;

FIG. 19 is a plan diagram showing a twelfth embodiment of the presentinvention;

FIG. 20 is a cross-sectional diagram taken along line XX--XX of FIG. 19;

FIG. 21 is a cross-sectional diagram taken along line XXI--XXI of FIG.19;

FIG. 22 is a plan diagram showing a thirteenth embodiment of the presentinvention;

FIG. 23 is a cross-sectional diagram taken along line XXIII--XXIII ofFIG. 22;

FIG. 24 is a plan diagram showing a fourteenth embodiment of the presentinvention;

FIG. 25 is a cross-sectional diagram taken along line XXV--XXV of FIG.24; and

FIG. 26 is a cross-sectional diagram of a conventional silicon thin filmtransistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

In the following description, embodiments of the present invention willbe explained with reference to the drawings.

Embodiment 1

Referring to FIGS. 1 and 2, an insulating substrate 1 using glass or thelike is provided, and a gate electrode 2 formed of Cr (chromium) isformed thereon. A gate insulating layer 3 formed of silicon nitride orsilicon oxide is formed on substrate 1 and gate electrode 2, with afirst impurity contained silicon layer 4 formed of amorphous siliconcontaining a suitable amount of impurities which become a donor or anacceptor formed thereon. An intrinsic silicon layer 5 formed ofamorphous silicon which becomes an active layer is formed on insulatinglayer 3 and silicon layer 4, with a protective insulating layer 6 formedof silicon nitride or silicon oxide being deposited thereon. A sourceelectrode 7 and a drain electrode 8 formed of ITO (indium tin oxide) arealso provided, and a contact part 9 of first impurity contained siliconlayer 4 connects first impurity contained silicon layer 4 to sourceelectrode 7 and drain electrode 8.

An explanation will now be given of the production method by referringto FIGS. 2(a) to 2(c).

As shown in FIG. 2(a), gate electrode 2 is formed on insulatingsubstrate 1, gate insulating layer 3 is formed on insulating substrate 1containing gate electrode 2, and first impurity contained silicon layer4 is formed on gate insulating layer 3. The pattern of the pair of firstimpurity contained silicon layers is formed by etching layer 4 in such amanner as to transversely cross the terminal part of gate electrode 2.

As shown in FIG. 2(b), intrinsic silicon layer 5 and protectiveinsulating layer 6 are continuously formed. Specifically, by use of aphotoresist as a mask, protective insulating layer 6 is etched by usinga buffered fluoric acid solution. Intrinsic silicon layer 5 is etched byuse of a dry etching method using a CF gas to expose first impuritycontained silicon layer 4, and after cleaning the surface of exposedfirst impurity contained silicon layer 4 with a diluted fluoric acidaqueous solution, the photoresist is peeled off. In such a manner asdescribed above, intrinsic silicon layer 5 and protective insulatinglayer 6 are of the same shape and are formed on gate insulating layer 3and on the pair of first impurity contained layers 4.

As shown in FIG. 2(c), source electrode 7 and drain electrode 8 areformed in such a manner as to contact the contact part 9 of exposedfirst impurity contained silicon layer 4.

By means of the above-described procedures, the silicon thin filmtransistor shown in FIG. 2(c) is obtained.

In the present embodiment, since intrinsic silicon layer 5 is formed onfirst impurity contained silicon layer 4, and protective insulatinglayer 6 is formed on intrinsic silicon layer 5, the Junction state offirst impurity contained silicon layer 4 to intrinsic silicon layer 5 isimproved, and good transistor characteristics can be obtained.

Also, although first impurity contained silicon layer 4 and intrinsicsilicon layer 5 are discontinuously formed in both the prior art and thepresent invention, it has been experimentally proven that in the case ofusing an amorphous silicon layer on first impurity contained siliconlayer 4 and intrinsic silicon layer a better junction can be obtained byforming intrinsic silicon layer 5 on first impurity contained siliconlayer 4 than by forming first impurity contained silicon layer 4 onintrinsic silicon layer 5. Therefore, the present invention is betterthan the prior art.

Embodiment 2

Referring now to FIGS. 3 and 4, there is shown a second embodiment ofthe present invention.

The present embodiment provides an arrangement in which intrinsicsilicon layer 5 is formed within the boundary defined by gate electrode2,

In such a silicon thin film transistor, and particularly in whichamorphous silicon is used in intrinsic silicon layer 5, the off-currentis increased when light is irradiated onintrinsic silicon layer 5.

In the present embodiment, since intrinsic silicon layer 5 has beenformed within the boundary of gate electrode 2, light from the side ofinsulating substrate 1 is completely shut off and such light does notreach intrinsic silicon layer 5. Therefore the increase of current dueto light irradiation can be reduced to a large extent.

It is noted that the present embodiment ca only be realized by changingthe mask pattern in contrast to the case of the first embodiment.

Embodiment 3

Referring now to FIGS. 5 and 6, there is shown a third embodiment of thepresent invention.

In this embodiment, a contact part 9 is formed outside of the boundarydefined by gate electrode 2.

In the first embodiment, as shown in FIG. 2(c), since contact part 9 isformed in overlapping relation to the terminal part of gate electrode 2,the overlapped width of gate electrode 2 and first impurity containedsilicon layer 4 is 2L. However, the design rule is that the width shouldbe equal to L. On the other hand, in the present embodiment, as shown inFIG. 6, the overlap width becomes L. As is well known, the increase ofthe overlap capacity due to such overlap retards the response speed.

Therefore, according to the present embodiment, since the overlap widthis half of that of the first embodiment, an improvement of the responsespeed of the transistor can be devised.

It is noted that the present embodiment can be realized only by changingthe mask pattern.

Embodiment 4

Referring now to FIGS. 7 and 8, there is shown a fourth embodiment ofthe present invention.

In the present embodiment, the terminal part of intrinsic silicon layer5 is positioned outside of the boundary defined by gate electrode 2 andwithin the boundary defined by first impurity contained silicon layer 4.

In the present embodiment, since light from the side of insulatingsubstrate 1 which reaches the terminal part of intrinsic silicon layer 5formed of the boundary defined by gate electrode 2, can be reduced to alarge extent over the whole range of such terminal part of intrinsicsilicon layer 5 by first impurity contained silicon layer 4, theoff-current of the transistor at the time of light irradiation can bedecreased to a large extent.

The present embodiment can be realized only by changing the maskpattern.

Embodiment 5

Referring now to FIGS. 9 and 10, there is shown a fifth embodiment ofthe present invention.

In the present embodiment, the counterposing edges of the pair of firstimpurity contained silicon layers 4 under intrinsic silicon layer 5 areformed in a comb teeth shape. Since first impurity contained siliconlayer 4 and intrinsic silicon layer 5 are formed discontinuously, asufficiently good junction might not be obtained, even if the junctionstate is improved in comparison with that of the prior art. When thejunction is insufficient, parasite resistance occurs in the junctionpart, and the on-current of the transistor is lowered.

In the present embodiment, since the counterposed edges of the pair offirst impurity contained silicon layers 4 are formed in uneven shapes,the parasite resistance of the junction part is decreased, and loweringof the on-current of the transistor can be prevented.

FIG. 9 shows the embodiment in which the teeth of opposing firstimpurity contained silicon layers 4 are in direct opposing relation,that is, in which a convex part of one first impurity contained siliconlayer 4 is counterposed to a convex part of the other first impuritycontained silicon layer 4, and a concave part of the one first impuritycontained silicon layer is counterposed to a concave part of the otherfirst impurity contained silicon layer 4. FIG. 10 shows the embodimentin which a concave part of one first impurity contained silicon layer 4is counterposed to a convex part of the other first impurity containedsilicon layer 4, that is, in which the teeth of opposing first impuritycontained silicon layers 4 are offset from each other.

The present example can be realized only by changing the mask pattern incontrast to the above-described embodiments.

Embodiment 6

Referring now to FIG. 11, there is shown a sixth embodiment of thepresent invention.

An explanation of the production method will be given by referring toFIGS. 11(a) to 11(c).

As shown in FIG. 11(a), gate electrode 2 having light shieldingproperties is formed on insulating substrate 1 having a lighttransparency property, and gate insulating layer 3 is formed oninsulating substrate 1 containing gate electrode 2. First impuritycontained silicon layer 4 is formed on gate insulating layer 3, and byetching first impurity contained silicon layer 4, the pattern of a pairof first impurity contained silicon layers 4 is formed in such a manneras to transversely cross the terminal part of gate electrode 2.

Intrinsic silicon layer 5 and protective insulating layer 6 are formedin succession on gate insulating layer 3 and first impurity containedsilicon layer 4, and a photoresist 11 is coated on protective insulatinglayer 6. Ultraviolet rays 12 are then irradiated from the back surfaceof insulating substrate 1 to expose photoresist 11 by use of gateelectrode 2 as a mask.

As shown in FIG. 11(b), photoresist 11, which is coordinated to gateelectrode 2, is formed by developing the exposed photoresist 11, andprotective insulating layer 6 and intrinsic silicon layer 5 are thenetched by using this photoresist 11 as a mask to form intrinsic siliconlayer 5 and protective insulating layer 6 coordinated to gate electrode2 and connecting the pair of impurity contained silicon layers 4.

As shown in FIG. 11(c), photoresist 11 is then peeled off and sourceelectrode 7 and drain electrode 8 are formed in such a manner as tocontact the contact parts of the pair of first impurity containedsilicon layers 4.

In the silicon thin film transistor produced by the procedures describedabove, since the pattern of intrinsic silicon layer 5 and protectiveinsulating layer 6 is formed by utilizing the pattern of gate electrode2, the number of photomasks can be reduced for one sheet in comparisonwith that in the case of the first embodiment.

Since first impurity contained silicon layer 4 absorbs ultraviolet rays12, it is preferable in the present embodiment to make the filmthickness of first impurity contained silicon layer 4 as thin aspossible.

Embodiment 7

Referring now to FIG. 12, there is shown a seventh embodiment of thepresent invention.

An explanation will now be given of the production method by referringto FIGS. 12(a) to 12(c).

As shown in FIG. 12(a), gate electrode 2 having light shieldingproperties is formed on insulating substrate 1 having a lighttransparency property, and gate insulating layer 3 is formed oninsulating substrate 1 containing gate electrode 2. First impuritycontained silicon layer 4 is formed on gate insulating layer 3, andfirst impurity contained silicon layer 4 is then etched to form thepattern of a pair of first impurity contained silicon layers 4 in such amanner as to transversely cross the terminal part of gate electrode 2.

Intrinsic silicon layer 5 and protective insulating layer 6 are formedin succession on gate insulating layer 3 and first impurity containedsilicon layer 4, and photoresist 11 is coated on protective insulatinglayer 6. Ultraviolet rays are then irradiated from the back surface ofinsulating substrate 1 to expose photoresist 11 by using gate electrode2 and first impurity contained silicon layer 4 as masks.

As shown in FIG. 12(b), by developing photoresist 11, photoresist 11,which is coordinated to gate electrode 2 and first impurity containedsilicon layer 4, is formed. Protective insulating layer 6 and intrinsicsilicon layer 5 are then etched by use of this photoresist 11 as a maskto form intrinsic silicon layer 5 and protective insulating layer 6coordinated to gate electrode 2 and first impurity contained siliconlayer 4, and connecting the pair of first impurity contained siliconlayers 4.

As shown in FIG. 12(c), photoresist 11 is then peeled off, and sourceelectrode 7 and drain electrode 8 are formed in such a manner that theycontact the contact part 9 of the terminal part of the pair of firstimpurity contained silicon layers 4.

In the silicon thin film transistor produced in the above-describedprocedure, since the pattern of intrinsic silicon layer 5 and protectiveinsulating layer 6 is formed by utilizing the pattern of gate electrode2 and first impurity contained silicon layer 4, the number of photomaskscan be reduced to one sheet for the first embodiment.

Since it is required that ultraviolet rays are sufficiently absorbed byfirst impurity contained silicon layer 4, it is preferable in thepresent embodiment to make the film thickness of first impuritycontained silicon layer 4 as thick as possible.

Embodiment 8

Referring now to FIG. 13, there is shown an eighth embodiment of thepresent invention. In the present embodiment, an auxiliary electrode 10is formed in such a manner as to contact first impurity containedsilicon layers 4 and to cover the step difference part of sourceelectrode 7 and drain electrode 8 at the terminal part of first impuritycontained layer 4.

According to the present embodiment, even if source electrode 7 or drainelectrode 8 is broken at the step difference part, electrical connectioncan be obtained with auxiliary electrode 10.

For auxiliary electrode 10, it is satisfactory that Ti (titanium) or thelike is used.

Embodiment 9

Referring now to FIG. 14, there is shown a ninth embodiment of thepresent invention. In FIG. 14, there are provided insulating substrate1, gate electrode 2 using Cr (chromium), gate insulating layer 3 usingsilicon nitride, first impurity contained silicon layer 4 containingn-type impurities such as phosphorous or the like in amorphous silicon,intrinsic silicon layer 5, which becomes an active layer using amorphoussilicon, a second impurity contained silicon layer 4a on intrinsicsilicon layer 5 and containing impurities of the type reverse to theimpurities in first impurity contained-silicon layer 4, that is,impurities of the p-type such as boron or the like, protectiveinsulating layer 6 thereover and using silicon nitride or the like, andsource electrode 7 and drain electrode 8 using ITO (indium tin oxide) ora metal.

An explanation will now be given of the production method by referringto FIGS. 14(a) to 14(d).

As shown in FIG. 14(a), gate electrode 2 is formed on insulatingsubstrate 1, gate insulating layer 3 is formed on insulating substrate1, on which gate electrode 2 is formed, and on gate insulating layer 3,there is formed the pair of first impurity contained silicon layers 4 insuch a manner that they overlap gate electrode 2.

As shown in FIG. 14(b), intrinsic silicon layer 5, second impuritycontained silicon layer 4a, and protective insulating layer 6 arecontinuously formed. For forming second impurity contained silicon layer4a, a plasma CVD method, optical CVD method, ion implanting method usingboron or the like, electrical discharge method by use of a discharge ofa gas such as B₂ H₆ or the like can be used.

As shown in FIG. 14(c), by successively etching protective insulatinglayer 6, second impurity contained silicon layer 4a and intrinsicsilicon layer 5 by use of the same mask, these three layers are formedin a pattern of the same shape. In this case, pattern formation iscarried out such that intrinsic silicon layer 5 connects the pair offirst impurity contained silicon layers 4.

As shown in FIG. 14(d), source electrode 7 and drain electrode 8 to beconnected to the pair of impurity contained silicon layers 4 are thenformed.

In the present embodiment, since intrinsic silicon layer 5 and secondimpurity contained silicon layer 4a are continuously formed, thejunction interface of these two layers is extremely clean.

Further, since second impurity contained silicon layer 4a and protectiveinsulating layer 6 are also formed continuously, a clean interface canbe obtained.

It is noted that first impurity contained silicon layer 4 may be made asa p-type, and second impurity contained silicon layer 4a as an n-type.

Embodiment 10

Referring now to FIGS. 15 and 16, application of the silicon thin filmtransistor of the present invention to an active matrix type liquidcrystal display device will now be described.

In the present embodiment, a gate wiring 13 is formed at the same timeas the formation of gate electrode 2, and a source wiring 14 and pictureelement electrode 15 are formed at the same time as the formation ofsource electrode 7 and drain electrode 8. Therefore, the formation canbe carried out without increasing fundamentally the number of masksheets.

Cr or the like can be used for gate electrode 2 and gate wiring 13, andITO or the like for source electrode 7, drain electrode 8, source wiring14, and picture element electrode 15.

Embodiment 11

Referring now to FIGS. 17 and 18, there is shown an eleventh embodimentof the present invention. Specifically, the present embodiment providesfirst impurity contained silicon layer 4 in parallel to source wiring14, and as to all other aspects, it is the same as those in the tenthembodiment.

Embodiment 12

Referring now to FIGS. 19, 20, and 21, there is shown a twelfthembodiment of the present invention. The present embodiment is alsodirected to the application of the silicon thin film transistor of thepresent invention to an active matrix type liquid crystal displaydevice.

As will first be described with respect to the above-described tenthembodiment, there are various problems.

In the production procedure of FIG. 2(b) in the first embodiment, it isnecessary to clean the exposed surface of first impurity containedsilicon layer 4 by use of a diluted fluoric acid aqueous solution. Atthis time, gate insulating layer 3 is also etched. Moreover, sincesilicon nitride and silicon oxide used in gate insulating layer 3 arereadily etched by the diluted fluoric acid aqueous solution, the filmthickness of gate insulating layer 3 becomes considerably thin due tothe etching. Therefore, in the tenth embodiment, poor insulation is aptto occur between gate wiring 13 and source wiring 14 at the crossingpart of gate wiring 13 with source wiring 14.

Therefore, in the present embodiment, as shown in FIGS. 19 and 20, athird impurity contained silicon layer 4b is provided to make itdifficult for malinsulation to occur. Third impurity contained siliconlayer 4b is formed at the same time as first impurity contained siliconlayer 4, and is formed in such a manner as to cover gate insulatinglayer 3 positioned at the crossing part of gate wiring 13 with sourcewiring 14. Due to such an arrangement, gate insulating layer 3 coveredwith third impurity contained silicon layer 4b is not etched at the timeof cleaning the exposed surface of first impurity contained siliconlayer 4 by use of diluted fluoric acid aqueous solution, so that thefilm thickness thereof does not become thin, as shown in FIG. 20.

Embodiment 13

Referring now to FIGS. 22 and 23, there is shown a thirteenth embodimentof the present invention.

In the present embodiment, third impurity contained silicon layer 4b isformed along approximately the whole region defined by the boundary ofsource wiring 14 and is of a wider width than that of source wiring 14,so that the same effect as that achieved in the twelfth embodiment canbe obtained.

In the twelfth embodiment, as shown in FIG. 20, source wiring 14 isrequired to transversely cross the step difference between thirdimpurity contained silicon layer 4b and gate insulating layer 3generated by etching of the diluted fluoric acid solution. Accordingly,wire breaking of source wiring 14 is apt to occur at this stepdifference.

In the present embodiment, as shown in FIG. 23, among theabove-described step differences, since the step difference of gateinsulating layer 3 generated by etching width diluted fluoric acidsolution is absent, wire breaking of source wiring 14 at this stepdifference can be reduced.

Furthermore, in the present embodiment, since third impurity containedsilicon layer 4b absorbs light, and light leakage is reduced, thecontrast of the display is improved when the present embodiment is usedin an active matrix type liquid crystal display device.

Embodiment 14

Referring now to FIGS. 24 and 25, there is shown a fourteenth embodimentof the present invention.

In the present embodiment, third impurity contained silicon layer 4b isformed on gate insulating layer 3 over almost the whole range defined bythe boundary of source wiring 14 and is of a wider width than that ofsource wiring 14. Moreover, the pattern of intrinsic silicon layer 5aand protective insulating layer 6a is formed on third impurity containedsilicon layer 4b in such a manner as to cover the crossing part of gatewiring 13 and source wiring 14, and the same effect as that achieved inthe thirteenth embodiment can be obtained.

Furthermore, in the present embodiment, since intrinsic silicon layer 5aand protective insulating layer 6a are formed separately from gateinsulating layer 3 and third impurity contained silicon layer 4b at thecrossing part of gate wiring 13 with source wiring 14, poor insulationcharacteristics between gate wiring 13 and source wiring 14 can bereduced to a large extent.

Intrinsic silicon layer 5a and protective insulating layer 6a will beeffective at the same time as the formation of intrinsic silicon layer 5and protective insulating layer 6.

In the above-described embodiments 1-14, it is preferable to use siliconnitride, silicon oxide, or a multilayered film of silicon nitride andsilicon oxide for gate insulating layer 3. Although the use of anamorphous silicon layer is desirable for each impurity contained siliconlayer and intrinsic silicon layer, a polycrystalline silicon or the likecan also be used. For protective insulating layer 6, silicon nitride,silicon oxide, or the like can be used.

According to the present invention, the junction state of impuritycontained silicon layer 4 and intrinsic silicon layer can be improved,and good transistor characteristics can be obtained

In addition, the following effects can be obtained.

In the embodiments in which intrinsic silicon layer 5 is formed insideof the boundary defined by gate electrode 2, off-current at the time oflight irradiation can be decreased to a large extent.

In the embodiments in which the contact part 9 is formed at the outsideof the boundary defined by gate electrode 2, since the overlap capacitycan be reduced, the response properties of the transistor can beenhanced.

In the embodiments in which the terminal part of intrinsic silicon layer5 is positioned at the outside of the boundary define by gate electrode2 and inside of the boundary defined by impurity contained silicon layer4, off-current at the time of light irradiation can be reduced to alarge extent.

In the embodiments in which the counterposing edges of the pair ofimpurity contained silicon layers 4 under intrinsic silicon layer 5 areformed with comb teeth, the parasite resistance is reduced and loweringof the on-current of the transistor can be prevented.

In the production method in which the photoresist on protectiveinsulating layer 6 is exposed from the back surface of insulatingsubstrate 1, and the pattern of intrinsic silicon layer and protectiveinsulating layer 6 is formed by the pattern of the photoresist obtainedby developing this photoresist, the number of mask sheets can bedecreased, and cost reduction and improvement of the yield can beobtained.

In the embodiments in which second impurity contained silicon layer 4acontaining impurities of the type reverse to the impurities in firstimpurity contained silicon layer 4 of silicon is provided, since secondimpurity contained silicon layer 4a and protective insulating layer 6are continuously formed, a clean interface can be obtained.

Also, in the embodiments in which the silicon thin film transistors ofthe present invention are provided in an array-like arrangement, anexcellent silicon thin film transistor array having the respectivefeatures can be obtained.

In the embodiments in which, in the silicon thin film transistor array,third impurity contained silicon layer 4b has been followed in such amanner as to cover gate insulating layer 3 positioned at the crossingpart of gate wiring 13 with source wiring 14, it is difficult to formpoor insulation characteristics at this crossing part.

Further, in such silicon thin film transistor array in which thirdimpurity contained silicon layer 4b has been formed over approximatelythe whole region defined by the boundary of source wiring 14, with awider width than that of source wiring 14, malinsulation between gatewiring 13 and source wiring 14 can be reduced to a large extent.

What we claim is:
 1. A silicon thin film transistor comprising:aninsulating substrate; a gate electrode formed on the insulatingsubstrate and having edges defining a first external boundary; a gateinsulating layer formed on said insulating substrate containing saidgate electrode; a pair of first impurity containing silicon layersformed on said gate insulating layer in such a manner as to transverselycross said first boundary of said gate electrode; an intrinsic siliconlayer formed on said pair of first impurity containing silicon layersand on said gate insulating layer between said pair of first impuritycontaining silicon layers in such a manner as to interconnect said pairof first impurity containing silicon layers, said impurity containingsilicon layers having edges defining second external boundaries, wherebysaid first and second boundaries together define a third externalboundary; a protective insulation layer formed on said intrinsic siliconlayers and having the same shape as that of said intrinsic siliconlayer; and a source electrode and a drain electrode formed at contactparts of said pair of first impurity containing silicon layers, whereinall edges of said intrinsic silicon layer are positioned entirely withinsaid third boundary defined by edges of said gate electrode and saidfirst impurity containing silicon layers.